Add exception description on PDT parsing error.

Assets/Cryville/Crtr/Ruleset.cs

@@ -25,7 +25,7 @@ namespace Cryville.Crtr {
 		public void LoadPdt(DirectoryInfo dir) {
 			using (StreamReader pdtreader = new StreamReader(dir.FullName + "/" + data + ".pdt", Encoding.UTF8)) {
 				var src = pdtreader.ReadToEnd();
-				Root = new RulesetInterpreter(src, null).Interpret();
+				Root = (PdtRuleset)new RulesetInterpreter(src, null).Interpret();
 			}
 		}
 	}